Fiber lasers can be used in many applications. Fiber lasers are more compact, more reliable, and more efficient relative to solid state lasers, and can provide robust single-mode output, making them suitable for high energy and/or high power laser systems that generate optical beams for industrial, military, and long range lidar/ladar applications, for example.
An example of a fiber laser architecture is the semi-guiding high aspect ratio core (SHARC) architecture. Detailed descriptions of the SHARC architecture can be found in U.S. Pat. Nos. 8,606,062 and 7,983,312. FIG. 1A shows a transverse cross-sectional view of an example of a SHARC fiber laser. The SHARC fiber laser 100 includes a high aspect ratio core 110, signal claddings 120, a pump cladding 130 (also referred to as a mode index matching cladding), and a coating 140. The core 110 is sandwiched between the two signal claddings 120. The core 110 can have an aspect ratio between approximately 30:1 and 100:1, depending on the application. The core 100 includes an active dopant material. The active dopant material can be distributed in the core 110 according to a desired transverse distribution profile to control the gain profile within the core. The active dopant material can be active dopant ions (active lasing dopant ions or active amplifying dopant ions), such as Ytterbium ions, Erbium ions, Thulium ions, or a combination of two or more of these ions. The core 110 and the signal claddings 120 are surrounded by the pump cladding 130. The coating 140 covers the pump cladding 130. The coating 140 may be a polymer coating or any other flexible coating. For a high aspect ratio core 110, the expanded or “long” dimension is traditionally referred to as the slow axis, while the narrow dimension is referred to as the fast axis, as indicated in FIG. 1A.
The use of a planar structure for the core 110 allows for an increase in the core cross-sectional area, and therefore the laser output power, without a corresponding increase in signal intensity. The pump cladding 130 and coating 140 need not be planar and can maintain a more conventional circular shape even for a high aspect ratio planar core 110. However, it is common for the entire SHARC fiber 100 to be planar, as shown in FIG. 1A. Thus, the SHARC fiber laser 100 has a ribbon-like geometry, allowing it to be coiled in a spiral conformation around a carrier 150, as shown in FIG. 1B. The carrier 150 can include a cooling system or cooling structure for cooling the SHARC fiber laser 100. In the example shown in FIG. 1A, the core 100 is centered within SHARC fiber laser 100, such that the vertical axis AA divides the SHARC fiber laser 100 into two substantially identical symmetrical halves, and the horizontal axis BB similarly divides the SHARC fiber laser 100 into two substantially identical symmetrical halves. However, the core 110 and signal claddings 120 need not be centered within the SHARC fiber laser 100, and can be offset from either axis AA or axis BB.
A conventional circular fiber laser having a circular core is fed by a circular Gaussian beam. In contrast, the SHARC fiber laser 100 is fed with an elliptical beam due to the high aspect ratio core. Several free-space optics methods are known to be effective for changing a beam format from a collimated round shape to a collimated elliptical shape. For example, one or more cylindrical lenses can be used to achieve beam formatting and free-space optical coupling of an initially circular beam into a planar fiber waveguide. A disadvantage of free-space optical coupling methods however, is that the optical components are bulky and require precise alignment. U.S. Pat. No. 7,860,360 discloses a modified monolithic step-index slab/planar waveguide structure configured to convert circular Gaussian beams into elliptical Gaussian beams. The structure can be “spliced” between a circularly symmetrical step-index core fiber and a SHARC fiber laser and used to convert Gaussian modes of the step-index fiber into elliptical Gaussian modes mode-matched to the fundamental modes of the SHARC fiber laser.